Oman later devoted two quite substantial volumes to the same subject in the 1920s. However, this is still very much worth the read for those who need a quick overview of the subject. I have both this and the 2-volume set in hard copy, and peruse both on a regular basis.

This little book is only "young Charles Oman's" initial treatment of a subject so vast that he later (in the 1920's) devoted two largish volumes to it. (which work remains a principle "target" and benchmark to modern medieval military scholars)

Although this little book is a fun overview it is not remotely in the same class as the later fulsome work.

If the reader can obtain the two volume work this book can be passed over entirely....

Though Oman gives it a good try. There simply may not be room to give someone used to modern armies any idea of how completely amateur a mess antique and medieval ones were by comparison. He may point out how useless 10,000 burghers may be, except to form a wall of flesh, but he forgets to note that they are a positive burden the second they are out of sight from home because you have to come up with food for them. So this is not thick with the fictitious European "peasant levies" that never existed, except as a fossil of law that created a means to tax peasants to support a war when legal systems of taxation barely existed.

On the whole, a goodish introduction to the subject, after which you can get hold of Hans Delbrueck on the subject. Most modern writers, being stuffed full of legalisms with no real idea of the logistics of a pre-mechanized army or navy, are not as good. As well, his writing is clear, not out to impress you but inform you, with an appreciation for the grim humour of some situations.

"Nothing could show the primitive state of the military art better than the fact that generals solemnly sent and accepted challenges to meet in battle at a given place and on a given day. Without such precautions there was apparently a danger lest the armies should lose sight of each other, and stray away in different directions. When maps were non-existent, and geographical knowledge both scanty and inaccurate, this was no inconceivable event. Even when two forces were actually in presence, it sometimes required more skill than the commanders owned to bring on a battle."

If you don't want to wait for the "read on-line" page to load, here's the TOC:

CHAPTER I. THE TRANSITION FROM ROMAN TO MEDIAEVAL FORMS IN WAR (A.D. 378-582).
Disappearance of the Legion. Constantine's reorganization. The German tribes. Battle of Adrianople. Theodosius accepts its teaching. Vegetius and the army at the end of the fourth century. The Goths and the Huns. Army of the Eastern Empire. Cavalry all-important . . . 3-14

CHAPTER II. THE EARLY MIDDLE AGES (A.D. 476-1066).
Paucity of Data for the period. The Franks in the sixth century. Battle of Tours. Armies of Charles the Great. The Franks become horsemen. The Northman and the Magyar. Rise of Feudalism. The Anglo-Saxons and their wars. The Danes and the Fyrd. Military importance of the Thegnhood. The House- Carles. Battle of Hastings. Battle of Durazzo 15-27

CHAPTER III. THE BYZANTINES AND THEIR ENEMIES (A.D. 582-1071).
1. Character of Byzantine Strategy.
Excellence of the Byzantine Army. Scientific study of the art of war. Leo's ' Tactica.' Wars with the Frank. With the Turk. With the Slav. With the Saracen. Border warfare of Christendom and Islam. Defence of the Anatolic Themes. Cavalry as a defensive force. Professional and unchivalrous character of Byzantine officers . . 28-38
2. Arms, Organization, and Tactics of the Byzantines.
Reorganization of the Army of the Eastern Empire By Maurice. Its composition. Armament of the Horseman, A.D. 600-l000. Armament of the Infantry. Military Train and Engineers. The Officers. Cavalry tactics. Leo's ideal line of battle. Military Machines and their importance . . 38-48

CHAPTER IV.THE SUPREMACY OF FEUDAL CAVALRY (A.D. 1066-1346).
Unscientific nature of feudal warfare. Consequences of head-long charges. Tactical arrangements. Their primitive nature. Non-existence of strategy. Weakness of Infantry. Attempts to introduce discipline. Rise of Mercenaries. Supreme importance of fortified places. Ascendency of the defensive. The Mediaeval siege. Improvement of the Arts of Attack and Defence of fortified places. General character. The Crusades ... . 49-61

CHAPTER V. THE SWISS (A.D. 1315-1515).
i. Their Character, Arms, and Organization.
The Swiss and the Ancient Romans. Excellence of system more important than excellence of generals. The column of pikemen. The halberdier. Rapidity of the movements of the Swiss. Defensive armour. Character of Swiss armies 62-69
2. Tactics and Strategy.
The 'Captains' of the Confederates. The Echelon of three columns. The 'Wedge' and the 'Hedgehog' formations 70-73
3. Development of Swiss Military Supremacy.
Battle of Morgarten. Battle of Laupen. Battle of Sempach. Battle of Arbedo. Moral ascendency of the Swiss. Battle of Granson. Battle of Morat. Wars of the last years of the fifteenth century 73-87
4. Causes of the Decline of Swiss Ascendency.
The tactics of the Swiss become stereotyped. The Landsknechts and their rivalry with the Swiss. The Spanish Infantry and the short sword. Battle of Ravenna. Fortified Positions. Battle of Bicocca. Increased use of Artillery. Battle of Marignano. Decay of discipline in the Swiss Armies and its consequences 87-95

CHAPTER VI. THE ENGLISH AND THEIR ENEMIES (A.D. 1272-1485).
The Long-bow and its origin, Welsh rather than Norman. Its rivalry with the Cross-bow. Edward I and the Battle of Falkirk. The bow and the pike. Battle of Bannockburn and its lessons. The_French Knighthood and the English Archery. Battle of Cressy Battle of Poictiers. Du Guesclin and the English reverses. Battle of Agincourt. The French wars, 1415-1453. Battle of Formigny. Wars of the Roses. King Edward IV and his generalship. Barnet and Tewkesbury. Towton and Ferrybridge . 96-123

CHAPTER VII. CONCLUSION.
Zisca and the Hussites. The Waggon-fortress and the tactics depending on it. Ascendency and decline of the Hussites. Battle of Lipan. The Ottomans. Organization and equipment of the Janissaries. The Timariot cavalry. The other nations of Europe. Concluding remarks . . 124-134